A longitudinal voltage source interconnects into a first line and a second line and feeds a longitudinal voltage into each of the two lines. The voltage source has first and second H-bridge circuits, each with four switches, and with outer terminals and center terminals. The center terminals are connectable to disconnected locations of the first and second lines. A capacitor has a first capacitor terminal connected to the two first output terminals of the two H-bridge circuits and a second capacitor terminal connected to the two second output terminals of the two H-bridge circuits. One or more switching modules are connected between the first capacitor terminal and the first output terminals of the first and second H-bridge circuits, and one or more switching modules are connected between the second capacitor terminal and the second output terminals of the first and second H-bridge circuits.